1. Field of the Invention
The present invention relates to defect detection on a sample using time delay lock-in thermography (LIT) to ensure high throughput and improve defect detection sensitivity in production environments. Dark field illumination can be used to minimize background noise in certain LIT embodiments.
2. Related Art
During the manufacturing process samples may develop localized electrical defects that cause current leakage. Exemplary samples could include photovoltaic materials (e.g. 156 mm×156 mm wafers or 2160 mm×2460 mm panels), semiconductor wafers, or printed circuit boards (PCBs). Electrical defects, such as shunts and localized weak diodes, leak current and therefore can reduce the efficiency of the sample or even jeopardize the functioning of the devices on the sample. Therefore, it is highly desirable to accurately detect the positions of such electrical defects.
Defects have high current density passing through them and therefore heat up to a higher temperature than that of the sample. These temperature changes can be detected in the image from a focal plane array (FPA) IR camera. However, the change in temperature at a defect may be 5 orders of magnitude smaller than the background in the image. Thus, separating the defects from background noise may be challenging.
Lock-in thermography (LIT) is one known method for locating such defects. In LIT, the sample is modulated, e.g. by direct current injection into the sample or by photocurrent generated from illumination of the sample. When the modulation is by illumination, the method is sometimes called illuminated lock-in thermography (ILIT). Temperature changes caused by heating of the sample from the injected current or photocurrent are modulated at the same frequency. With either form of modulation, multiple frames of IR images are captured while the sample remains stationary.
Due to the shot noise of background IR radiation from the sample at room temperature as well as the very small temperature difference between the defects and the rest of the sample, and the limited dynamic range of the IR imaging sensor, a large number of images of the same field of view (FOV) are needed to average out the background noise, thereby improving the signal to noise ratio (SNR). Although the captured images are taken from the identical spatial location, they are a function of time as the temperature of the sample oscillates at the frequency of modulation. In a typical embodiment, the images are filtered by multiplying each image by a weighting factor that varies sinusoidally in time at the same frequency as the modulation or “lock-in” frequency. In general, the improvement of SNR is proportional to the square root of N, wherein N is the total number of frames.
Conventional LIT requires that the sample remains stationary while the IR camera acquires the necessary number of images for lock-in averaging. If the size of the sample is greater than the field of view (FOV) of the camera, the sample (or the IR camera) needs to move to a completely different location to capture a new set of IR images after one set of images is captured for one location on the sample. Unfortunately, this stop-go time as well as the settling time (which includes repositioning with its attendant velocity ramp up and ramp down) takes a large portion of the total inspection time, especially for very large samples that can be greater than 2 m×2 m in size, thereby undesirably reducing throughput. This overhead in conventional lock-in thermography becomes a significant limiting factor of inspection throughput.
Therefore, a need arises for a technique of detecting defects on a sample that increases inspection throughput compared to conventional LIT while maintaining its accuracy.